Special Print, Manufacturing, From Sensor to Transmitter · 2015. 7. 30. · shear effect Point of...
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Manufacturing From Sensor to Transmitter Robust Industrial Sensors through Integral Electronics A. Kirchheim, D. Otter, D. Kobler, K. Bräuer, A. Lehmann, G. Schaffner Kistler Group, Winterthur ifm2007 – International Mechatronics Forum 12 + 13 September 2007 Winterthur, Switzerland Special Print 920-361e-05.08 Modelling Mechatronics Mechanical Engineering Information Technology Electrical Engineering Actuators and Sensors Computerized Process Engineering Electric Engineer M
Transcript of Special Print, Manufacturing, From Sensor to Transmitter · 2015. 7. 30. · shear effect Point of...
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Manufacturing
From Sensor to Transmitter
Robust Industrial Sensors through Integral Electronics A. Kirchheim, D. Otter, D. Kobler, K. Bräuer, A. Lehmann, G. SchaffnerKistler Group,Winterthur
ifm2007 –International Mechatronics Forum12 + 13 September 2007Winterthur, Switzerland
Special Print920-361e-05.08
Modelling
Mechatronics
MechanicalEngineering
InformationTechnology
Electrical Engineering
Actuators and Sensors
Computerized Process
Engineering
ElectricEngineer
M
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From Sensor to Transmitter
Robust Industrial Sensors through Integral Electronics
A. Kirchheim, D. Otter, D. Kobler, K. Bräuer, A. Lehmann, G. SchaffnerKistler Group, Eulachstr. 22, 8408 Winterthur, Switzerland
1 IntroductionThe high resolution, wide measuring range and substantial overload protection of piezoelectric sensors make them par-ticularly suitable for monitoring and analyzing manufacturing processes. New, compact and powerful electronic components are increasingly allowing integration of the evaluation electron-ics directly into the sensor. Ongoing miniaturization makes it possible today to use electronics allowing very flexible pro-gramming in new force and strain transmitters. This enables the major advantages of piezoelectric sensors and simple industrial sensors to be combined.
2 Piezoelectric Measuring ChainPiezoelectric force and strain sensors are now used not only in very many areas of research but also increasingly in produc-tion, either directly or as OEM components in machinery. They capture dynamic changes up to extremely high frequencies and additionelly allow successful quasistatic measurement. These multifaceted applications include measuring the joining forces involved in press fitting, clinching or riveting.
Such manufacturing applications make use of press force sen-sors, a common piezoelectric type, as shown in Figure 1a and 1b. The special, patented press force design is characterized by axial preloading of the sensor. This is required to ensure good linearity of the output signal, particularly for small compres-sion forces. The press force sensor contains an annular pair of quartz washers as an integral sensor element sensitive to pres-sure. The pressure-sensitive direction of the washers is oriented so that a charge signal proportional to the magnitude of the compression force is outputed and converted in a charge meter (charge amplifier) into a proportional electrical voltage. As a result of the piezoelectric measuring principle and high rigid-ity of the press force sensor, measurement is virtually without deflection.
For ease of mounting e.g. in press plungers the sensor has adapter flanges and a special mounting thread.
unloaded loaded
Crystal lattice
Crystal elements
Fig. 1a: Longitudinal piezoelectric effect
Mounting thread KInternal centering seats D2Mounting thread PFree passage D3
Connector protectionPlug connectionPiezo element
Fig. 1b: Design of a preloaded force link (press force sensor)
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unloaded loaded
Principle ofshear effect
Point of application of force
Fig. 2a: Piezoelectric shear effect
10 kN
Fig. 2b: Design of a piezoelectric strain sensor
Like strain gages, strain sensors can measure surface strain directly proportionally to the load on machine structures. This strain is transferred by friction to a shear-sensitive sensor element integral with the body of the sensor. This is achieved by attaching the sensor to the structure with an M6 screw tightened to 10 kN in order to measure strains up to 800 με in loaded machine structures.
In the case of the sensors described above, the charge signal proportional to strain (and hence force) is conditioned in an external charge amplifier and passed to the measuring PC, PLC or other control unit. Figure 3 shows the piezoelectric measuring chain consisting of sensor, cable and industrial charge amplifier including block diagram.
For use in the industrial manufacturing environment the multi-channel industrial charge amplifier (ICAM: Industrial Charge Amplifier Manufacturing) is integrated into a robust, sealed case (IP65), can be flexibly adapted for different processes and has sufficiently high accuracy and linearity. Other fea-tures include:
Wide, adjustable measuring range (±100 ... 1 000 000 pC)• Integral peak memory with programmable functionality• Robust, serial communication interface (RS-232C)• Offset, gain and filter values via setup selectable menu• Automatic zero compensation•
The heart of the charge amplifier (Q/U converter) is imple-mented as a thick layer ceramic module. The complete PCB is a multilayer Starrflex manufactured from FR4 on which SMDs are mounted (Figures 4 and 5). It has an 8-bit microcontroller with integral 12-bit A/D converter (25 MIPS; 12-bit ADC, 100 ksps; 12-bit DAC; 32 kB flash; TQFP64).
However, the force can also very easily be represented indi-rectly through the strain in the loaded structure (see Figures 2a and 2b).
Fig. 3: Piezoelectric measuring chain consisting of sensor, cable and charge amplifier (signal conditioning)
Sensor Cable
Q/U converter
Reset
Q Ct
Ri Cc
Cr
Rt
Uo
Press Force SensorMeasuring range 70 kN
Strain Sensor Connecting Cable Adapter ICAM
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Fig. 4: Block diagram of ICAM charge amplifier
Fig. 5: Starrflex PCB of charge amplifier (folded)
3 Miniaturization and Integration of ElectronicsTo meet the demand for greater integratability and robust design, the electronic signal conditioning system was incor-porated into the actual case of the sensor. The result is a homogeneous measuring system with a high degree of inte-gration with a rugged connecting cable (M12 connector) for use in tough production environments. The entire measuring chain is combined and moved to the actual measuring point and form a system called a transmitter (Strain transmitter Type 9238A..., press force transmitter Type 9337A...) as shown in Figure 6.
The electronics of the charge amplifier underwent continu-ous development to meet the more stringent miniaturization requirements imposed on transmitter electronics.• The Q/U converter module was integrated directly onto
the substrate (laminate)• Circuit complexity was reduced to that necessary (custom-
ized)• The mechanical design was optimized
The transmitter electronics use SMD technology on a rigid multilayer PCB manufactured from special (highly insulated) FR4 laminate. Here as well the heart of the electronics is an 8-bit microcontroller with integral 12-bit A/D converter (100 MIPS; 12-bit ADC, 100 ksps; 12-bit DAC; 128 kB flash; TQFP64). All this is implemented on two PCB surfaces in the form of a disk just 25 mm in diameter (corresponding to 980 mm2; see Figure 6).
The miniaturization has brought a greater than 3-fold reduc-tion in the area required for each channel, yet digitization of the signal (block diagram in Figure 7) has also improved flexibility. More flexible signal scaling or inversion allowed by the digitization is achieved at the cost of increased process-ing power (digital signal processing) requirements, which can however be provided very cost-effectively these days with more powerful microcontrollers.
Fig. 6: Standard (piezoelectric) measuring chain in contrast with transmitter
ICAM inside Strain Transmitter Charge Amplifier
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4 Configuration and MeasurementAll of the presented transmitters can also be configured through an RS-232C interface. Default settings and adjust-ments can be made using either a terminal program or the much more user-friendly PC tool "ManuWare".
Fig. 8: Kistler products currently supported by ManuWare
Strain transmitter
Reference measuring chain for force
Fig. 11: The strain at the transmitter corresponds to the force read off the reference measuring instrument. Scaling is performed in this case using the teach-in technique.
Charge Amplifier Force Strain Electromechanical ICAM Transmitter Transmitter NC joining module (piezo)
Setup
Fig. 9: Visualization of measurands and indication of signal status are im por tant aids whenever commissioning
Fig. 10: Self-explanatory setup procedures lead the user through the entire process of setting the transmitter. Here the mapping of the mechanical measurand (force in N) to the electrical output signal (voltage in mV) is specified.
However, although appropriate for force sensors this method is scarcely suitable for strain sensors, as the relationship between force and resultant strain is usually unknown. In this case the second method, the teach-in technique, leads to the desired outcome (see Figure 11).
ManuWare basically supports those Kistler products that can be set up via serial interface and where measurement data can be transmitted by analog or serial means.
A useful feature of this program is the measurement data visualization and export function (in Figure 9), which repre-sents another step along the path of implementing Kistler's measure and analyze concept. Measurands, for example, can be displayed as a function of time and saved in common export formats*. A box also indicates the logical status of all of the control signals to and from the transmitter.
Different methods of scaling the measuring range are offered. One is to merely type in the sensor data from the calibration record. There is no need to introduce forces or other mechanical variables (shwon in Figure 10) here.
*) In preparation
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Displacement sensor
Direct force measurement
Indirect force measurement
Direct force measurementForce shuntmeasurement
Fig. 12: Measurement of press forces (mounting)
A distinction is drawn between direct, indirect and shunt force measurement. With the direct method, the entire force to be recorded is passed through the sensor for very sensi-tive high-resolution measurement. The indirect approach measures the force through the resultant strain, and the force shunt technique positions the sensor in the structure so that only part of the force passes through the sensor. Indirect and force shunt measurement trade very high resistance to overloading off against lower sensitivity. As joining processes are usually tracked by monitoring the joining force through the resultant displacement, the latter also has to be measured (Figure 13).
The next two figures show typical force curves as generated by the described force and strain transmitters during press fitting or resistance welding. The transmitters have electronic systems whose power, ease of use and flexibility was previ-ously only encountered in laboratory measurement.
Fig. 13: Press force transmitter mounted in press. Typical press fitting operation against shoulder.
Fig. 14: Bench machine for resistance welding with mounted strain sensors on the electrode and strain transmitters on the frame of the machine; clamping force (quasistatic) and welding force (dynamic) over a welding cycle.
6 Summary New, compact and powerful electronic components are in creasingly allowing integration of the evaluation electronics directly into the sensor. This means that these days electron-ics allowing very flexible programming can be used in new force and strain transmitters, making it possible to combine the major advantages of piezoelectric sensors and simple industrial sensors.
7 ReferencesBill, B.: Messen mit Kristallen: Grundlagen und Anwen-• dungen der piezoelektrischen Messtechnik. Moderne Indus-trie, Landsberg/Lech: 2002Tichy, J.; Gautschi, G.: "Piezoelektrische Messtechnik" Phy-• sikalische Grundlagen Kraft-, Druck- und Beschleunigungsaufnehmer, Verstärker, Springer-Verlag, Berlin Heidelberg, New York, 1980Kirchheim A., Lehmann A., Schaffner G., Deuerling, R., • Thommes H.; Hussmann D.: Sichere Verbindungen durch Überwachung der Fügekräfte, 13th Paderborn Symposium: Fügetechnik Mechanisches Fügen und Kleben, Paderborn, 29 & 30 November 2006
5 ApplicationsThe high resolution, wide measuring range and substantial overload protection make piezoelectric sensors ideal for mon-itoring and analyzing manufacturing processes. However, it is not enough to rely on the performance of the sensor itself, the mounting point in the path of the forces through the machine must also be correct. Figure 12 shows mounting examples in a (C-frame) press as frequently used in assembly, for example for press fitting.
Build-up of additional force (re-pressing)
Mechanical stop (shoulder)
Push-in
Sens
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Time [s]
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Strain TransmitterType 9238A...
Strain SensorType 9237A...
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